I start by 12 months to discourage mouthing of toys. Basically once the first 4 teeth have come through the need to chew for teeth is less. Yes some kids have a real oral fixation and if you take the toys away they will resort to fingers. But you can limit them to a toy.

For me it is also about the other kids and the fact that if you chew on a toy long enogh the face disappears or there is too much wear and tear on the figure. Since the toys need to last for several years worth of kids it isn't fair for one child to destroy them. I have some infant toys and as long as a child spends most of their day chewing those are the only things they may play with. Gradually they get a few blocks,etc. that can stand up to the teeth.

I start by doing the switch - take away the toy they are mouthing but shouldn't and give them an infant toy or remove the toy and move the child over to where the bin of mouthable toys are - usually this one. After awhile they respond to "not in your mouth" said in a firm just loud enough to get point across that I am serious.

By 18 months it isn't an option. No toys at my house are allowed in the mouth.

Since I have been starting earlier to ban mouthing toys I have actually seen less biting take place and assuming they are getting the concept that nothing but food I serve goes in the mouth.